The core shroud in boiling water reactors (BWR) supports and locates the reactor core within the reactor pressure vessel (RPV), and forms the flow partition for the reactor core coolant. It is constructed of a number of stainless steel circular rings and cylindrical rolled plate sections, joined at their ends with circumferential welds. The welding introduces residual stresses in the weld heat affected zones. It additionally locally sensitizes the stainless steel, which depletes the grain structure of chromium and reduces corrosion resistance. These factors, combined with the BWR reactor coolant environment, make the weld heat affected zones susceptible to intergranular stress corrosion cracking (IGSCC), observed in many BWR shrouds. The cracking impairs the structural integrity of the shroud. Particularly, lateral seismic loading or loss of coolant accident (LOCA) conditions could cause relative displacements at cracked weld locations which could produce large core flow leakage and misalignment of the core that could prevent control rod insertion and safe shutdown.
IGSCC cracking in BWR core shrouds has typically been addressed by installation of a tie rod design shroud repair. The repair assembly integrates the required vertical and lateral restraint features to replace the structural function of all the shroud circumferential weld joints, assuming their failure. This repair uses tensioned tie rods to compressively load the respective circumferential weld joints, preventing their separation. Horizontal stabilizers are attached to the tie rod assemblies which maintain the lateral alignment of the top and bottom of the core at the top guide and core plate, respectively. Additionally, limit stops are located to restrain the shroud cylinder sections below the top guide and core plate support rings. The required anchorage at the bottom of the shroud is obtained by machining a hole through the shroud support, which allows an attachment such as a clevis and toggle bar, reacting against the bottom of the shroud support plate. Another anchorage secures the lower end of the tie rod to a gusset plate, in turn, secured to the shroud support plate. These anchorage arrangements are not feasible where the structural integrity of the shroud support plate has also been impaired by IGSCC cracking, such that it cannot support the additional localized loading for the required tie rod anchorage at the bottom of the shroud.